The present invention relates to a method of manufacturing a semiconductor device, more specifically, a method of manufacturing a semiconductor device in which quantum dots are formed by self-assembled growth.
Recently, semiconductor devices using quantum dots, e.g., quantum dot lasers including quantum dots formed in the active layers of the semiconductor lasers, etc. are much studied.
As a technique of forming the quantum dots used in the quantum dot laser, etc. is known a technique of forming the quantum dots by self-assembled growth. This technique utilizes the phenomena that a lattice misfit semiconductor is grown by vapor phase epitaxy to thereby form self-assembled quantum dots of the three-dimensional fine structure.
For the self-assemble of the quantum dots, several modes are known. Among them, the most popular mode is a mode called Stranski-Krastanow mode (S-K mode). S-K mode is a mode that the epitaxially grown semiconductor crystals grow two-dimensionally (into film) at the start of the growth but grow three-dimensionally on the stage where the film has exceeded the elastic limit. A film whose lattice constant is larger than that of a material below is epitaxially grown to thereby be self-assembled quantum dots, i.e., three-dimensional grown islands.
However, the conventional quantum dot laser having the quantum dots formed by self-assembled growth has a quantum dot density of below 5×1010 cm−2 including 5×1010 cm−2 and, because of such insufficient density, disadvantageously cannot make high-speed direct modulation of above 10 Gbps including 10 Gbps. Then, to improve the density of the quantum dots, new methods of growing the quantum dots are proposed. As one of such new methods of growing the quantum dots is a method of irradiating Sb to a base semiconductor layer for the quantum dots to be grown on before the quantum dots are grown.
The conventional method of growing the quantum dots including the step of irradiating Sb to the base semiconductor layer for the quantum dots to be grown on will be explained with reference to FIGS. 31A-31D. In the following, quantum dots of InAs are grown on a base semiconductor layer of GaAs.
First, Sb is irradiated to the surface of the base layer 100 of GaAs. Thus, on the surface of the base layer 100, an Sb layer 102 of substantially a monolayer (ML) is formed (FIG. 31A).
Then, a quantum dot layer 104 of InAs is formed by, e.g., MOCVD on the base layer 100 with the Sb layer 102 on the surface. The lattice constant of InAs is different from that of GaAs forming the base layer 100, and quantum dots 106 are formed in the quantum dot layer 104 by S-K mode (FIG. 31B).
While the quantum dot layer 104 is growing, the Sb on the surface of the base layer 100 diffuses mutually with the InAs forming the quantum dot layer 104 to be deposited on the surface of the quantum dot layer 104. Thus, on the surface of the quantum dot layer 104, a surface layer containing the Sb, e.g., an InSb layer 108 is formed (FIG. 31C).
Next, on the quantum dot layer 104 with the InSb layer 108 formed on the surface, a capping layer 110 of GaAs is grown by, e.g., MOCVD, burying the quantum dots 106 (FIG. 31D).
The background arts of the present invention are disclosed in, e.g., Japanese published unexamined patent application No. 2005-093553, Japanese published unexamined patent application No. 2004-335665 and Japanese published unexamined patent application No. Hei 6-204498.
It is known that the method of growing the quantum dots described above with reference to FIGS. 31A-31D can increase the density of the quantum dots about double in comparison with the conventional methods. On the other hand, the method has a disadvantage that the crystal quality of the quantum dots is degraded, and the emission efficiency of the quantum dots is lowered.
Furthermore, in the quantum dot laser using the quantum dot layer as the active layer, quantum dot layers are laid to form the active layer. However, when a number of the laid quantum dot layers is large, the quantum dot laser has disadvantages that strains of the lower quantum dot layers are accumulated, and the upper quantum dot layers are largely different from the lower quantum dot layers in the size distribution and the density of the quantum dots, and defects take place, and other disadvantages.
In the conventional quantum dot laser, the cladding layer must be grown at a relatively low growth temperature of, e.g., about 600° C. so that the quantum dots may not be degraded. The growth of the cladding layer at such low growth temperature increases the roughness of the interface between the cladding layer and semiconductor layers, such as the SCH (Separate Confinement Heterostructure) layer, the active layer, the contact layer, etc., which are adjacent to the cladding layer. Resultantly, a disadvantage that the scattering loss of light is increased takes place.